1. Field of the Invention
The present invention relates to a sensor utilizing attenuated total reflection (hereinafter referred to as ATR), such as a surface plasmon resonance sensor for quantitatively analyzing a substance in a sample by utilizing excitation of a surface plasmon, and more particularly to a sensor, utilizing ATR, of a type that detects a dark line occurring in a reflected light beam due to ATR by the use of photodetection means consisting of a plurality of light-receiving elements juxtaposed in a predetermined direction.
2. Description of the Related Art
In metals, if free electrons are caused to vibrate in a group, compression waves called plasma waves will be generated. The compression waves generated in a metal surface are quantized and called a surface plasmon.
A variety of surface plasmon resonance sensors have been proposed for quantitatively analyzing a substance in a sample by taking advantage of a phenomenon that a surface plasmon is exited by light waves. Among such sensors, one employing a system called xe2x80x9cKretschmann configurationxe2x80x9d is particularly well known (e.g., see Japanese Unexamined Patent Publication No. 6(1994)-167443).
The surface plasmon resonance sensor employing the xe2x80x9cKretschmann configurationxe2x80x9d is equipped mainly with a dielectric block formed, for example, into the shape of a prism; a metal film, formed on a surface of the dielectric block, for placing a sample thereon; a light source for emitting a light beam; an optical system for making the light beam enter the dielectric block at various angles of incidence so that the condition for total internal reflection is satisfied at the interface between the dielectric block and the metal film; and photodetection means for detecting the state of the surface plasmon resonance, that is, the state of ATR by measuring the intensity of the light beam satisfying total internal reflection at the interface.
In order to obtain various angles of incidence, as described above, a relatively thin light beam may be caused to strike the above-mentioned interface at different angles of incidence, or relatively thick convergent or divergent rays may be caused to strike the interface so that they contain components incident at various angles. In the former, the light beam whose reflection angle varies with a change in the incidence angle of the incident light beam can be detected by a small photodetector that is moved in synchronization with the variation in the reflection angle, or by an area sensor extending in the direction in which the angle of reflection varies. In the latter, on the other hand, rays reflected at various angles can be detected by an area sensor extending in the direction in which all of the reflected rays can be received.
In the surface plasmon resonance sensor mentioned above, where Ksp represents the wave number of the surface plasmon, xcfx89 represents the angular frequency of the surface plasmon, c represents the speed of light in vacuum, and xcex5m and xcex5s represent the dielectric constants of the metal and the sample, respectively.
If the dielectric constant xcex5s of the sample is found, the density of a specific substance in the sample is found based on a predetermined calibration curve, etc. As a result, by finding the incidence angle xcex8sp at which the intensity of reflected light drops, the dielectric constant of the sample, that is, the properties of the sample related to the refractive index thereof can be specified.
In this kind of surface plasmon resonance sensor, photodetection means in the form of an array can be employed with the object of measuring the aforementioned incidence angle xcex8sp with a high degree of accuracy and in a large dynamic range, as disclosed in Japanese Unexamined Patent Publication No. 11(1999)-326194. The photodetection means is formed by a plurality of light-receiving elements juxtaposed in a predetermined direction. The light-receiving elements are disposed to respectively receive the components of a light beam satisfying total internal reflection at various angles of reflection at the aforementioned interface.
In that case, differentiation means is provided for differentiating the photodetection signals output by the light-receiving elements of the aforementioned photodetection if a light beam strikes the metal film at a specific incidence angle xcex8sp equal to or greater than a critical angle of incidence at which total internal reflection takes place, evanescent waves having electric field distribution are generated in the sample in contact with the metal film, whereby a surface plasmon is excited at the interface between the metal film and the sample. When the wave vector of the evanescent light is equal to the wave number of the surface plasmon and therefore the wave numbers between the two are matched, the evanescent waves and the surface plasmon resonate and light energy is transferred to the surface plasmon, whereby the intensity of light satisfying total internal reflection at the interface between the dielectric block and the metal film drops sharply. The sharp intensity drop is generally detected as a dark line by the above-mentioned photodetection means.
Note that the above-mentioned resonance occurs only when the incident light beam is a p-polarized light beam. Therefore, in order to make the resonance occur, it is necessary that a light beam be p-polarized before it strikes the interface.
If the wave number of the surface plasmon is found from a specific incidence angle xcex8sp at which ATR takes place, the dielectric constant of a sample can be obtained by the following Equation:
Ksp(xcfx89)=(xcfx89/c){xcex5m(xcfx89)xcex5s}1/2/{xcex5m(xcfx89)+xcex5s}1/2
means, in the direction in which the light-receiving elements are juxtaposed. The properties of the sample related to the refractive index thereof are often analyzed based on differentiated values output by the differentiation means, particularly the differentiated value corresponding to a dark line that occurs in a reflected light beam.
In addition, a leaky mode sensor is known as a similar sensor making use of ATR, as disclosed, for instance, in xe2x80x9cSpectral Researches,xe2x80x9d Vol. 47, No.1 (1998), pp. 21 to 23 and pp. 26 and 27. The leaky mode sensor is constructed mainly of a dielectric block in the form of a prism, for example; a cladding layer formed on a surface of the dielectric block; an optical waveguide layer, formed on the cladding layer, for placing a sample thereon; a light source for emitting a light beam; an optical system for making the light beam enter the dielectric block at various angles of incidence so that the condition for total internal reflection is satisfied at the interface between the dielectric block and the cladding layer; and photodetection means for detecting the excited state of the waveguide mode, that is, the state of ATR by measuring the intensity of the light beam satisfying total internal reflection at the interface between the dielectric block and the cladding layer.
In the leaky mode sensor with the construction mentioned above, if a light beam falls on the cladding layer through the dielectric block at angles of incidence equal to or greater than an angle of incidence at which total internal reflection takes place, the light beam is transmitted through the cladding layer and then only light with a specific wave number, incident at a specific angle, is propagated in the optical waveguide layer in a waveguide mode. If the waveguide mode is excited in this manner, the greater part of the incident light is confined within the optical waveguide layer, and consequently, ATR occurs in which the intensity of light satisfying total internal reflection at the above-mentioned interface drops sharply. Since the wave number of light propagating in the optical waveguide layer depends on the refractive index of the sample on the optical waveguide layer, the refractive index of the sample and/or the properties of the sample related to the refractive index thereof can be analyzed by finding the above-mentioned specific angle of incidence at which ATR takes place.
The leaky mode sensor also can employ the aforementioned photodetection means in the form of an array in order to detect the position of a dark line occurring in the reflected light by ATR. In addition, the aforementioned differentiation means is often employed along with the photodetection means.
In the field of pharmaceutical research, etc., the above-mentioned surface plasmon resonance sensor and leaky mode sensor are sometimes used in a random screening method of finding a specific substance that couples with a desired sensing medium. In this case, a sensing medium is placed on the aforementioned thin film layer (i.e., the metal film in the case of the surface plasmon resonance sensor, or the cladding layer and the optical waveguide layer in the case of the leaky mode sensor), and various solutions of substances (liquid sample) are added to the sensing medium, and each time a predetermined time elapses, the aforementioned differentiated value is measured. If the added substances are coupled with the sensing medium, the refractive index of the sensing medium varies with the lapse of time by the coupling. Therefore, by detecting the above-mentioned differentiated value every time a predetermined time elapses and then judging whether or not the differentiated value has been varied, it can be judged whether or not the added substances and the sensing medium have been coupled, that is, whether or not the added substances are specific substances that couple with the sensing medium. In this case, both the sensing medium and the liquid sample are samples to be analyzed. As such a combination of specific substances and a sensing medium, there is, for example, a combination of an antigen and an antibody.
In the above-mentioned surface plasmon resonance sensor and leaky mode sensor, incidentally, the properties of a sample related to the refractive index thereof have been found based on values obtained by differentiating the photodetection signals output from the light-receiving elements of the photodetection means. Because of this, in the case of measuring an incidence angle xcex8sp at which ATR takes place, for example, the width of a dark line corresponding to the incidence angle xcex8sp is very narrow. When the width of the dark line is narrower than the pitch between two adjacent light-receiving elements of the photodetection means, a difference in the incidence angle xcex8sp, which corresponds to a difference in the position of incidence of a dark line whose width is less than the pitch between the light-receiving elements, cannot be detected and therefore an error in detection becomes great. In the case of measuring a change with the lapse of time in the incidence angle xcex8sp at which ATR takes place, the change with lapse of time in the incidence angle xcex8sp cannot be detected if a change in the incidence position of a dark line corresponding to the change with lapse of time in the incidence angle xcex8sp is less than the pitch between the light-receiving elements. As a result, an error in detection becomes great and it becomes difficult to make an accurate analysis of a sample.
The present invention has been made in view of the circumstances mentioned above. Accordingly, it is the primary object of the present invention to provide a sensor, utilizing ATR, which is capable of adjusting the width of a dark line in a reflected light beam which corresponds to ATR, enhancing accuracy of detection, and making an accurate analysis of a sample.
To achieve this end and in accordance with an important aspect of the present invention, there is provided a sensor utilizing attenuated total reflection, comprising:
a dielectric block;
a thin film layer, formed on a surface of the dielectric block, for placing a sample thereon;
a light source for emitting a light beam;
an optical system for making the light beam enter the dielectric block at various angles of incidence so that a condition for total internal reflection is satisfied at an interface between the dielectric block and the thin film layer;
photodetection means, comprising a plurality of light-receiving elements juxtaposed in a predetermined direction and disposed to respectively receive components of the light beam satisfying the total internal reflection condition at the interface, for detecting the attenuated total reflection;
differentiation means for differentiating photodetection signals output from the light-receiving elements of the photodetection means, in the predetermined direction in which the light-receiving elements are juxtaposed; and
adjustment means for optically expanding the width of a dark line, corresponding to the attenuated total reflection, of the light beam which falls on the photodetection means, so that the width of the dark line becomes greater than a pitch between the light-receiving elements.
In accordance with another important aspect of the present invention, there is provided a sensor utilizing attenuated total reflection, comprising:
a dielectric block;
a metal film, formed on a surface of the dielectric block, for placing a sample thereon;
a light source for emitting a light beam;
an optical system for making the light beam enter the dielectric block at various angles of incidence so that a condition for total internal reflection is satisfied at an interface between the dielectric block and the metal film;
photodetection means, comprising a plurality of light-receiving elements juxtaposed in a predetermined direction and disposed to respectively receive components of the light beam satisfying the total internal reflection condition at the interface, for detecting the attenuated total reflection caused by surface plasmon resonance;
differentiation means for differentiating photodetection signals output from the light-receiving elements of the photodetection means, in the predetermined direction in which the light-receiving elements are juxtaposed; and
adjustment means for optically expanding the width of a dark line, corresponding to the attenuated total reflection, of the light beam which falls on the photodetection means, so that the width of the dark line becomes greater than a pitch between the light-receiving elements.
In accordance with still another important aspect of the present invention, there is provided a sensor utilizing attenuated total reflection, comprising:
a dielectric block;
a cladding layer formed on a surface of the dielectric block;
an optical waveguide layer, formed on a surface of the cladding layer, for placing a sample thereon;
a light source for emitting a light beam;
an optical system for making the light beam enter the dielectric block at various angles of incidence so that a condition for total internal reflection is satisfied at an interface between the dielectric block and the cladding layer;
photodetection means, comprising a plurality of light-receiving elements juxtaposed in a predetermined direction and disposed to respectively receive components of the light beam satisfying the total internal reflection condition at the interface, for detecting the attenuated total reflection caused by excitation of a waveguide mode in the optical waveguide layer;
differentiation means for differentiating photodetection signals output from the light-receiving elements of the photodetection means, in the predetermined direction in which the light-receiving elements are juxtaposed; and
adjustment means for optically expanding the width of a dark line, corresponding to the attenuated total reflection, of the light beam which falls on the photodetection means, so that the width of the dark line becomes greater than a pitch between the light-receiving elements.
As described above, the three sensors according to the present invention is characterized by comprising the adjustment means for optically expanding the width of a dark line, corresponding to the attenuated total reflection, of the light beam which falls on the photodetection means, so that the width of the dark line becomes greater than a pitch between the light-receiving elements.
The width of the dark line refers to the width of a region where light intensity is reduced to more than half the difference between the minimum value of the light intensity in the dark line region and the light intensity in a region other than the dark line, when the components of the light beam are detected by the photodetection means. That is, the width of the dark line is equivalent to the reverse of the full width at half maximum intensity (FWHM).
The aforementioned sensors of the present invention may further comprise means for moving the adjustment means in and out of an optical path between the dielectric block and the photodetection means.
The aforementioned adjustment means can employ a diverging lens or diverging lens array. It can also employ a diffusing plate, a zoom lens, or the like.
The three sensors of the present invention may further comprise means for moving the photodetection means in the direction in which the light beam propagates.
In addition, the three sensors of the present invention may further comprise means for rotating the photodetection means on an axis substantially perpendicular to both the direction in which said light beam propagates and the predetermined direction in which the light-emitting elements are juxtaposed.
The differentiated value that is output from the aforementioned differentiation means, as it is, maybe displayed on display means and used for analyzing the properties of a sample. Based on the differentiated value, an incidence angle xcex8sp at which ATR takes place may be automatically calculated and displayed on display means. In addition, every time a predetermined time elapses, a quantity of change in the differentiated value can be calculated from the differentiated value. Based on the quantity of change, the properties of a sample can be analyzed. Furthermore, based on the incidence angle xcex8sp, as well as a predetermined calibration curve, etc., a quantitative analysis of a specific substance in a sample may be automatically made and displayed on display means in real time.
It is preferable that the aforementioned differentiation means be capable of calculating a difference between photodetection signals output from adjacent light-receiving elements of the photodetection means. The photodetection means can suitably employ, for instance, a photodiode array, etc.
According to the present invention, the width of a dark line, corresponding to ATR, of the light beam which falls on the photodetection means, is optically expanded by the adjustment means so that the width of the dark line becomes greater than the pitch between the light-receiving elements. The dark line occurring in the light beam is received by two or more light-receiving elements. The light-receiving elements that are receiving the dark line will receive light having a light quantity which corresponds to the incidence angle xcex8sp that is used for an analysis of the properties of a sample. Therefore, when differentiating the photodetection signals output from the light-receiving elements, in the direction in which the light-receiving elements are juxtaposed, and then detecting an incidence angle xcex8sp or a change with the lapse of time in the angle from the differentiated value, the accuracy of detection is enhanced and therefore an analysis of a sample can be accurately made.
There are cases where the width of a dark line occurring in a light beam varies with sample types, the wavelength of the light beam, etc. If there is provided means for moving the adjustment means in and out of the optical path between the dielectric block and the photodetection means, the adjustment means can be disposed between dielectric block and the photodetection means when it is necessary to expand a light beam to widen the dark line. When there is no need to expand a light beam, a reduction in the quantity of the light beam due to the adjustment means can be prevented by moving the adjustment means out of the optical path between the dielectric block and the photodetection means.
If a diverging lens or diffusing plate is used as the adjustment means, an increase in the cost will be slight. In addition, if a zoom lens is employed as the adjustment means, a light beam can be expanded with a desired magnification ratio in accordance with the width of a dark line occurring in the light beam.
According to the present invention, there is provided means for moving the photodetection means in a direction in which a light beam propagates. Therefore, the width of a dark line corresponding to ATR can be adjusted by moving the photodetection means in the direction in which a light beam propagates, by the moving means. That is, when it is necessary to expand the width of the dark line, it becomes possible to make the width of the dark line greater than the pitch between the light-receiving elements by moving the photodetection means away from the dielectric block. Because of this, the dark line can be received by two or more light-receiving elements. Therefore, the light-receiving elements that are receiving the dark line can receive light which has a light quantity corresponding to an incident angle xcex8sp that is used for an analysis of the properties of a sample.
Therefore, when differentiating the photodetection signals output from the light-receiving elements, in the direction in which the light-receiving elements are juxtaposed, and then detecting an incidence angle xcex8sp or a change with the lapse of time in the angle from the differentiated value, the accuracy of detection is enhanced and therefore an analysis of a sample can be accurately made. Since there is no need to provide an optical component, such as a lens, a diffusing plate, etc., between the dielectric block and the photodetection means, a reduction in the light intensity due to an optical component can be prevented. In addition, a magnification ratio for the width of a dark line on the photodetection means can be set as desired.
According to the present invention, there is provided means for rotating the photodetection means on an axis substantially perpendicular to both the direction in which the light beam propagates and the predetermined direction in which the light-emitting elements are juxtaposed. Therefore, the width of a dark line corresponding to ATR can be adjusted by rotating the photodetection means by the rotation means. For example, when it is necessary to expand the width of the dark line, the photodetection means is rotated so that a light beam falls obliquely on the photodetection means. This makes it possible to make the width of the dark line greater than the pitch between the light-receiving elements of the photodetection means. Because of this, the dark line can be received by two or more light-receiving elements. Therefore, the light-receiving elements that are receiving the dark line can receive light which has a light quantity corresponding to an incident angle xcex8sp that is used for an analysis of the properties of a sample. Thus, the section for measuring the light beam can be made structurally simple and compact. When differentiating the photodetection signals output from the light-receiving elements, in the direction in which the light-receiving elements are juxtaposed, and then detecting an incidence angle xcex8sp or a change with the lapse of time in the angle from the differentiated value, the accuracy of detection is enhanced and therefore an analysis of a sample can be accurately made.